1. Field of the Invention
This invention relates to a novel method for monitoring functional characteristics of an organ/tissue ("organ"), being stored in an ex vivo warm preservation process, intended for transplantation. More particularly, the method involves measuring one or more indicia of the organ function, as a means of assessing functional capabilities of the organ which can then be correlated with its posttransplantation course.
2. Description of the Background and Related Art
There continues to be an extreme shortage of organs for transplantation. For example, kidney transplantation is largely dependant upon the availability of organs retrieved from heart-beating cadaver donors. While awaiting transplantation, kidneys must be stored in a fashion that will result in the restoration of normal, immediate function. Therefore, the clinical preservation of organs is much more limited than the experimental models.
Additionally, a large and as yet untapped source of organs for transplantation are accident victims who succumb at the site of an injury and those having short post-trauma survival times. These accident victims are not used as organ donors because of the ischemic damage. Likewise, older potential donors are often considered borderline because of questions relating to organ function. Thus, the development of methods which will allow clinicians to evaluate prospectively the functional capabilities of an allograft ex vivo may allow for the expansion of the organ donor pool. Similarly, the current demand for organs cannot be met with existing technology. Unless new sources of organs can be developed, the number of transplantation procedures will remain constant. Additionally, the donor pool cannot be substantially expanded because there is no process/system available to evaluate if the borderline or warm ischemically damaged organ will function when transplanted; i.e. until now, the only means to evaluate the functional capabilities of an organ intended for transplantation was to transplant it.
There are two commonly used methods of storage for kidneys--both involving hypothermia: preservation by continuous hypothermic perfusion; and simple hypothermic storage (see for example Collins et al., 1969, Lancet 2:1219). While a variety of perfusates have been utilized clinically, these two methods of kidney storage have remained substantially unchanged for the past 20 years. The current perfusate solution that represents the state-of-the-art in hypothermic organ preservation, and provides for optimized organ preservation under hypothermic conditions, contains components which prevent hypothermic induced tissue edema; metabolites which facilitate organ function upon transplantation; anti-oxidants; membrane stabilizers; colloids; ions; and salts (Southard et al.,1990, Transpl. 49:251; and Southard, 1989, Transpl. Proc. 21:1195). The formulation of this perfusate is designed to preserve the organs by hypothermic induced depression of metabolism. While it minimizes the edema and vasospasm normally encountered during hypothermic storage, it does not provide for the utilization of a substantially expanded donor pool. This is due to the fact that an allograft, marginally damaged by warm ischemia cannot tolerate further damage mediated by the hypothermia.
Further, since preservation of organs using hypothermia reduces organ metabolism and oxidative needs, such preservation methods do not allow for measurement of functional characteristics which can be used prospectively to determine the function of the organ posttransplantation. For example, while at normal physiologic temperatures the phospholipids making up the cell membranes are highly fluid, under the hypothermic conditions utilized to preserve organs intended for transplantation, the lipid bilayer experiences a phase-change and becomes gel-like, with greatly reduced fluidity. The essentially frozen lipid in the cell membranes negates the utilization of 02, even in the presence of a high O.sub.2 -tension. The metabolic consequence is glycolysis, which is analogous to the state of anoxia. The hypothermic conditions utilized in organ preservation probably represents the reason why previous studies raising the O.sub.2 -tension did not uniformly demonstrate the benefit of increased oxygenation of tissues.
In organ metabolism, a direct relationship between oxygen requirements and temperature exists. In fact, hypothermia may exert a greater depression of oxidative metabolism in the kidney than in the body as a whole and may even help to explain the success in preserving kidney relative to other organs. A major portion of the oxygen consumed by the kidney is utilized for the process of active sodium reabsorption and reabsorption is by far the most important of all the tubular transport processes (Zerahn, 1956, Phsiol. Scand. 36:300). It has been described that below 18.degree. C., hypothermia inhibits the tubular activity of the kidney (Bickford et al., 1937, J. Physiol. 89:198) and that at 4.degree. C., the utilization of oxygen is approximately 5% of that at normothermia.
Hypothermic storage can further complicate assessment of function since it is not benign; i.e., it can produce vasospasm and subsequent edema in an allograft (Robison, 1953, Biol. Rev. 28:153; Rixon et al., 1954, Rev. Canc. Biol. 13:83). Hypo-thermically preserved organs can experience glomerular endothelial cell swelling and loss of vascular integrity along with tubular necrosis. These phenomenon can be attributed to the hypothermic conditions employed. Hypothermia can also inhibit the Na/K dependant ATPase and result in the loss of the cell volume regulating capacity. The loss of volume regulation is what causes the cellular swelling and damage. Without adequate oxygen delivery, the anoxia leads to disintegration of the smaller vessels after several hours of perfusion. The inability to adequately utilize oxygen, and the subsequent depletion of ATP stores, mean that anaerobic glycolysis is the principal source of energy under traditional preservation conditions. The lack of molecular oxygen for oxidative phosphorylation which occurs in either warm or cold ischemia, leads to the accumulation of NADH and the depletion of ATP stores within the mitochondria. The subsequent loss of nucleosides is probably a very important factor in the failure of tissues subjected to warm ischemia and prolonged periods of cold ischemia to regen- erate ATP after restoration of the blood supply. The inability to supply adequate oxygen has lead to the routine reliance on hypothermia for organ preservation.
Recently, processes/systems have been described which utilize a temperature range of 18.degree. C.-35.degree. C. for some form of organ preservation. However, none of these have established organ metabolism or function anywhere near the normal range. Nor have any of these described using the product of the organ metabolism during storage ex vivo or in situ, to establish functional capabilities which can be correlated with posttransplantation course.
The present inventor has developed a process/system to preserve organs ex vivo without traditional hypothermia (4.degree. C.-10.degree. C.) which has been described in detail previously (U.S. patent application Ser. Nos. 08/476,456 and 08/372,782, the disclosures of which are herein incorporated by reference). The process/system provides the necessary oxygen delivery, nutrients for metabolism, oncotic pressure, pH, perfusion pressures, and flow rates to support organ metabolism ex vivo, most often within or near the respective normal range in vivo. A near normal rate of metabolism is defined herein as about 70%-90% of the range of normal rates of metabolism. Further, the process/system supports a level of metabolism ex vivo which provides enough oxidative metabolism to result in the normal functional product of the organ. The development of this process/system which supports organs ex vivo, without traditional hypothermia, presents the opportunity to support a near normal rate of metabolism.